Re: Fly size and castability
The size of the fly definitely is the issue because of its lack of density. The flat surface on the front of that popper provides quite a bit of air resistance. The five weight line may not have enough oomph to to pull that popper through the air. Its kind of similar to the idea of running with an umbrella behind you. If you drop down to a size 10 popper, your casts should improve. (and you still catch PLENTY of sunfish)
Just for clarification, not all size 6 or 8 flies will have this affect on your casts. There is an ideal weight/density range which will cast well on a given weight of fly rod/line. A bead-head wooly bugger that weighed exactly the same as those size 6 poppers might cast just fine, due to its smaller profile. A size 6 Mickey Finn, with its slender profile and light weight, should cast just fine. If you were to bump up to a size 6 Clouser Minnow with lead eyes, that might bee too heavy and give you a different casting variance that didn't seem "right". It also might not feel "right" when you whacked yourself in the back of the head with the Clouser, as this happening is very common with Clousers on light, and especially short rods.